The present invention relates to electrical protection circuitry for magnetic disk drives and more particularly circuitry to protect against electrostatic discharge (ESD) and electrical overstress (EOS) in read/write transducers utilizing magnetoresistive read sensors.
Magnetic head disk drive systems have been widely accepted in the computer industry as a cost-effective form of data storage. In a magnetic disk drive system, a magnetic recording medium in the form of a disk rotates at high speed while a magnetic read/write transducer, referred to as a magnetic head, xe2x80x9cfliesxe2x80x9d slightly above the surface of the rotating disk. The magnetic disk is rotated by means of a spindle drive motor. The magnetic head is attached to or formed integrally with a xe2x80x9csliderxe2x80x9d which is suspended over the disk on a spring-loaded support arm known as the actuator arm. As the magnetic disk rotates at operating speed, the moving air generated by the rotating disk in conjunction with the physical design of the slider lifts the magnetic head, allowing it to glide or xe2x80x9cflyxe2x80x9d slightly above and over the disk surface on a cushion of air, referred to as an air bearing. The flying height of the magnetic head over the disk surface is typically only a few tens of nanometers or less and is primarily a function of disk rotation, the aerodynamic properties of the slider assembly and the force exerted by the spring-loaded actuator arm.
A major problem that is encountered during manufacture, handling and use of magnetic recording transducers, referred to as heads, is the buildup of electrostatic charges on the various elements of a head or other objects which come into contact with the heads, particular heads of the thin film type, and the accompanying spurious discharge of the static electricity thus generated. Static charges may be produced for example by the presence of certain materials, such as plastics, during manufacture and subsequent handling of the heads. These static charges arc across the edge of the insulating layer between the magnetic pole tips and adjacent conductive layers which are exposed and positioned adjacent to the transducing gap at the air bearing surface facing the recording medium thus causing erosion of the pole tips and degradation of the transducer in reading and writing of data.
Magnetoresistive sensors, also referred to as xe2x80x9cMR heads,xe2x80x9d are particularly useful as read elements in magnetic transducers, especially at high data recording densities. The MR sensor provides a higher output signal than an inductive read head. This higher output signal results in a higher signal-to-noise ratio for the recording channel and allows higher areal density of recorded data on a magnetic disk surface.
As described above, when an MR sensor is exposed to ESD, or even a voltage or current input larger than that intended under normal operating conditions, referred to as electrical overstress or EOS, the MR sensor and other parts of the head may be damaged. This sensitivity to electrical damage is particularly severe for MR read sensors because of their relatively small physical size. For example, an MR sensor used for extremely high recording densities will have a cross-section of 100 Angstroms (.ANG.) by 1.0 micrometers (um) or smaller. Discharge of voltages of only a few volts through such a physically small sensor, behaving like a resistor, is sufficient to produce currents capable of severely damaging or completely destroying the MR sensor. The nature of the damage which may be experienced by an MR sensor varies significantly, including complete destruction of the sensor via melting and evaporation, contamination of the air bearing surface, generation of shorts via electrical breakdown, and milder forms of damage in which the head performance may be degraded.
Prior solutions to ESD and EOS protection can be summarized into two types of approaches: 1) by using diode(s) and 2) by shorting out the MR sensor element. However, both of these approaches have significant disadvantages. In the diode approach, the diode is intended to remain in parallel with the MR sensor element during normal operation of the disk drive. The current flowing through the diode during normal operation must be small in order for the diode to not affect the operating effectiveness of the MR sensor element. Common bias voltages for MR heads are in the range of 350 mV to 700 mV, which is the regime over which a diode begins to conduct current. This leaking current through the diode leads to shot noise, which will lower the signal to noise ratio of the readback process. Diodes also introduce parasitic capacitance across the head, and from the head leads to the ground, which adversely affects the maximum readback bandwidth achievable with the head. Moreover, modern generations of Giant Magnetoresistive (GMR) heads are so sensitive to ESD, the diode is not sufficiently forward biased to shunt enough current away from the MR sensor element even for low ESD voltages.
Shorting out the MR sensor element, on the other hand, provides the best possible ESD protection. The problem with this technique is that it is no longer possible to test the MR head after the short is applied. Ideally, the short is applied early in the MR head fabrication process, and not removed until the disk drive is assembled. Due to the value added to the MR head as it goes through wafer fabrication, slider fabrication, suspension mounting, and head stack assembly (HSA) build, it is beneficial to be able to test the MR head at various points.
Elser et al. U.S. Pat. No. 4,317,149 discloses an inductive head having short discharge paths formed by the deposition of conductive material in recesses formed in an insulating layer so that the static electric discharge will occur in areas displaced from the critical pole tip and gap area at the slider air bearing surface. Schwartz et al. U.S. Pat. No. 4,800,454 discloses an inductive head assembly wherein the magnetic pole piece and the inductive coil winding are coupled to the slider to allow discharge of any static electric charges which may build up. The winding is connected to the slider body via a diode with high forward and reverse voltage drops, or through a fusible link.
U.S. Pat. No. 5,465,186 describes an approach for protecting a magnetic read/write transducer from the effects of electrical overstress and electrostatic discharge by shorting out the conductive leads of a magnetoresistive (MR) sensor element to provide a low resistance, conductive path that bypasses the MR element and minimizes electrical current through the MR sensing element during discharge of static electrical charge. The MR sensor lead terminal pads are shorted together by soldering or by using twisted conductor pairs. The other transducer elements such as the MR magnetic shields, the inductive coil and the inductive magnetic yoke structure may also be shorted to the MR sensor leads by soldering the lead terminal pads together at the slider surface. Remotely located protective devices, such as reversed diode pairs, can also be connected across the MR sensor element using the twisted pair. The short is removed prior to placing the MR head into operation in the magnetic storage system.
U.S. Pat. No. 5,491,605 describes a scheme for protecting a magnetic read/write transducer from EOS and ESD. The elements of the MR and inductive heads are shorted together and to the slider substrate by depositing a conductive material layer, such as tungsten, over the slider air bearing surface to provide a low resistance, conductive path that minimizes current through the MR element during discharge of electrostatic charge. The conductive layer is removed by wet etching prior to placing the magnetic head into operation in a magnetic storage system.
A switchable short was described in U.S. Pat. No. 5,465,186 that would allow the short to be temporarily opened for testing. However, this method is difficult to realize, as switches require large amounts of real estate on the back of the slider, and the switching process requires low resistance shorting and re-shorting structures. Switches can also be expected to last for only a limited number of opening and closing cycles.
A need therefore exists for providing an MR read/write head assembly with ESD and EOS shunt protection during manufacturing that would allow the MR read/write head to be tested at various manufacturing stages.
A principle objective of the present invention is to minimize the damage to an (G)MR sensor caused by the discharge of electrostatic discharge through or electrical overstress of the (G)MR sensor element, by using resistors to provide a low resistance bypass to the (G)MR sensor.
The other objective of the present invention is to provide a method for protecting the (G)MR sensor from ESD and EOS damage but at the same time allow the (G)MR sensor element to be tested at various stages in the manufacturing process.
In view of the foregoing objects, the present invention provides a low resistance bypass that shunts electrostatic discharge and electrical overstress from the (G)MR sensor element during a discharge. The low resistance bypass is achieved by connecting one or more resistors in series, across and in parallel to the (G)MR sensor element. The resistors, resistor leads, or both the resistors and leads are removed from across the head at an appropriate step during the manufacturing process.
The present invention also provides a method that would allow the (G)MR sensor element to be tested at the desired stages during the manufacturing process. By using appropriate electronic circuitry, the low resistance bypass can be temporarily disabled, i.e. no current is allowed to flow through it, thereby allowing the (G)MR sensor element to be tested.
In an embodiment of the present invention, thin film resistors are deposited during the (G)MR head fabrication across and in parallel to the (G)MR sensor element. These resistors are removed at a desired step during the manufacturing process. When testing is desired, an electronic circuitry can be externally connected to the low resistance bypass to temporarily disable the bypass. The removal of the electronic circuitry would re-enable the low resistance bypass.